Negative electrode material for lithium ion secondary battery, negative electrode for lithium ion secondary battery, and lithium ion secondary battery

ABSTRACT

A negative electrode material for a lithium ion secondary battery including carbon over a part or a whole of a surface of an oxide of silicon, in which the content of the carbon is from 0.5 mass-% to less than 5 mass-%.

TECHNICAL FIELD

The present invention relates to a negative electrode material for alithium ion secondary battery, a negative electrode for a lithium ionsecondary battery, and a lithium ion secondary battery.

BACKGROUND ART

Although presently graphite is mainly used as a negative electrodematerial for a lithium ion secondary battery, it has been known thatthere exists a theoretical capacity limitation of 372 mAh/g in dischargecapacity with respect to graphite. Since mobile devices, such as a cellphone, a notebook computer, and a tablet terminal, have come to havehigher performance in recent years, a demand for a higher capacitylithium ion secondary battery has become stronger, and a negativeelectrode material, which can attain still higher capacity of a lithiumion secondary battery, has been sought-after.

Consequently, development of a negative electrode material containing anelement, which has high theoretical capacity and ability for absorptionand desorption of a lithium ion (hereinafter also referred to as“specific element”, and that containing the specific element is alsoreferred to as “specific element substance”), has become active.

As the specific element, silicon, tin, lead, aluminum, etc. are wellknown. Among others, an oxide of silicon, which is one of the specificelement substances, has advantages over a negative electrode materialcomposed of other specific element substances, owing to higher capacity,lower cost, and better processibility, and negative electrode materialscontaining the same are especially energetically studied.

Meanwhile, the specific element substances are known to cause remarkablecubical expansion when alloyed by charging. Such cubical expansionmicronizes a specific element substance itself, and further destroys thestructure of a negative electrode material using the same, leading to abreakage of the electrical conductivity. Therefore it has a drawback inthat the capacity decreases significantly over cycles.

With respect to the drawbacks, for example, Japanese Patent No. 3952180discloses an electroconductive silicon complex for a negative electrodematerial for a nonaqueous electrolyte secondary battery, which ischaracterized in that a diffraction peak assignable to Si (111) isobserved in X-ray diffraction, the crystal size of silicon determined bythe Scherrer method based on the half width of the diffraction line isfrom 1 to 500 nm, and the surface of a particle having a structure wheresilicon crystallites are dispersed in a silicon compound is coated withcarbon.

Japanese Patent No. 3952180 claims that the technology thereof can yieldnot only surface electroconductivity but also a structure stable againstvolume change due to absorption and release of lithium, and as theresult improvement in long term stability and initial efficiency, bydispersing crystallites or fine particles of silicon in an inert rigidsubstance, for example, silicon dioxide, and fusing carbon over at leasta part of the surface thereof for imparting electrical conductivity.

Japanese Patent No. 4171897 discloses a negative electrode material fora nonaqueous electrolyte secondary battery characterized in that thematerial is an electroconductive powder composed of a material which canabsorb and release a lithium ion, and the surface of which is coatedwith a graphite film, and that the amount of the graphite coat is from 3to 40 weight-%, the BET specific surface area is from 2 to 30 m²/g, andthe graphite film shows spectra characteristic of a graphite structurenear 1330 cm⁻¹ and 1580 cm⁻¹ of Raman shift in a Raman spectrum.

Japanese Patent No. 4171897 claims that the technology thereof can yielda negative electrode for a lithium ion secondary battery which canachieve a quality level demanded from the market, by regulating physicalproperties of a graphite film coated on the surface of a material, whichcan absorb and release a lithium ion, within a specific range.

Japanese Patent Application Laid-Open (JP-A) No. 2011-90869 discloses anegative electrode material for a nonaqueous electrolyte secondarybattery, which is a negative electrode material to be used in a negativeelectrode for a secondary battery using a nonaqueous electrolyte,characterized in that the negative electrode material is composed of aparticle of silicon oxide expressed by a general formula of SiO_(x)whose surface is coated with a carbon film, and the carbon film istreated with a thermal plasma.

JP-A No. 2011-90869 claims that the technology thereof can yield anegative electrode material effective for a nonaqueous electrolytesecondary battery negative electrode, which has removed drawbacks ofsilicon oxide in expansion of an electrode and expansion of a battery bygas generation, and is superior in cycle performance.

SUMMARY OF INVENTION Technical Problem

However, when an oxide of silicon, which is one of the specific elementsubstances, is used as a negative electrode material, the initial chargeand discharge efficiency is low, and excessive battery capacity of apositive electrode is required for application to an actual battery, andtherefore a character of high capacity of an oxide of silicon has notbeen fully utilized in an actual lithium ion secondary battery accordingto conventional art. Further, as a negative electrode material to beapplied to a lithium ion secondary battery usable for a higherperformance mobile device, etc., it is necessary that the material cannot only store a large amount of lithium ions (namely the chargecapacity is high), but also release a larger amount of the storedlithium ions. Therefore, for a negative electrode material, which cancontribute to further improvement of lithium ion secondary batteryperformance, both of improvement of initial discharge capacity andimprovement of initial charge and discharge efficiency become important.

The present invention is made in view of the above needs, with an objectto provide a negative electrode material for a lithium ion secondarybattery, a negative electrode for a lithium ion secondary battery, and alithium ion secondary battery, which are superior in initial dischargecapacity as well as initial charge and discharge efficiency.

Solution to Problem

Specific means for achieving the object are as follows.

<1> A negative electrode material for a lithium ion secondary battery,including carbon over a part or a whole of a surface of an oxide ofsilicon, wherein the content of the carbon is from 0.5 mass-% to lessthan 5 mass-%.

<2> The negative electrode material for a lithium ion secondary batteryaccording to <1> above, wherein the carbon includes low crystallinitycarbon.

<3> The negative electrode material for a lithium ion secondary batteryaccording to <1> or <2> above, wherein a diffraction peak assignable toSi (111) is observed when the negative electrode material is subjectedto a powder X-ray diffraction (XRD) analysis.

<4> A negative electrode for a lithium ion secondary battery, including:

a current collector; and

a negative electrode material layer provided on the current collectorand including the negative electrode material according to any one of<1> to <3> above.

<5> A lithium ion secondary battery, including:

a positive electrode;

the negative electrode for a lithium ion secondary battery according to<4> above; and an electrolyte.

Advantageous Effects of Invention

The present invention can provide a negative electrode material for alithium ion secondary battery, a negative electrode for a lithium ionsecondary battery, and a lithium ion secondary battery, which aresuperior in initial discharge capacity as well as initial charge anddischarge efficiency.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an example of theconstitution of a negative electrode material according to theinvention.

FIG. 2 is a schematic cross-sectional view showing another example ofthe constitution of a negative electrode material according to theinvention.

FIG. 3 is a schematic cross-sectional view showing still another exampleof the constitution of a negative electrode material according to theinvention.

FIG. 4 is a schematic cross-sectional view showing still another exampleof the constitution of a negative electrode material according to theinvention.

FIG. 5 is a schematic cross-sectional view showing still another exampleof the constitution of a negative electrode material according to theinvention.

FIG. 6A is an enlarged cross-sectional view of a part of the negativeelectrode material according to FIG. 1 to FIG. 3, which is anillustrative diagram of an embodiment of the state of carbon 10 in thenegative electrode material.

FIG. 6B is an enlarged cross-sectional view of a part of the negativeelectrode material according to FIG. 1 to FIG. 3, which is anillustrative diagram of another embodiment of the state of carbon 10 inthe negative electrode material.

DESCRIPTION OF EMBODIMENTS

A numerical range expressed by “a to b” means herein a range defined bya and b as the minimum value and the maximum value respectively.

Further, with respect to the content of each component in a composition,if plural substances exist corresponding to a component in thecomposition, the content means herein, unless otherwise specified, thetotal amount of the plural substances existing in the composition.

<Negative Electrode Material for Lithium Ion Secondary Battery>

A negative electrode material for a lithium ion secondary batteryaccording to the invention (hereinafter occasionally abbreviated as“negative electrode material”) contains carbon over a part or a whole ofa surface of an oxide of silicon, wherein the content of the carbon isfrom 0.5 mass-% to less than 5 mass-%. With such a constitution,expansion and contraction on an occasion of absorption and release of alithium ion can be reduced, and decrease in capacity per unit mass of anoxide of silicon can be suppressed, and therefore the initial dischargecapacity and the initial charge and discharge efficiency can besuperior.

(Oxide of Silicon)

As an oxide of silicon according to the invention, any oxide containinga silicon element is usable, and examples thereof include siliconmonoxide (also called as “silicon oxide”), silicon dioxide, and siliconsuboxide. They may be used singly, or in a combination of plural kinds.

Although silicon oxide and silicon dioxide among oxides of silicon areexpressed generally as silicon monoxide (SiO) and silicon dioxide (SiO₂)respectively, they may be sometimes expressed by a compositional formulaSiOx (x is 0<x≦2) according to a found value (or a reduced value) ofcomposing elements depending on a surface condition (for example,presence of an oxidation film), or a formation condition of a compound,which are also understood as oxides of silicon according to theinvention. In this regard, the value of x can be calculated by analyzingquantitatively the oxygen content in an oxide of silicon, for example,by an inert gas fusion non-dispersive infrared absorption method.Further, in the event that a disproportionation reaction of an oxide ofsilicon (2SiO→Si+SiO₂) is included in a process for producing a negativeelectrode material according to the invention, the product may appear insome cases, due to a chemical reaction, in a state containing siliconand silicon dioxide (occasionally silicon oxide), which is alsounderstood as an oxide of silicon according to the invention.

Meanwhile, silicon oxide can be obtained, for example, by a publiclyknown sublimation process, by which a mixture of silicon dioxide andmetallic silicon is heated to form a gas of silicon monoxide, and thegas is cooled to deposit. Further, silicon oxide is available on themarket as silicon oxide, silicon monoxide, etc.

For a negative electrode material according to the invention, an oxideof silicon has preferably a structure, in which silicon crystallites aredispersed in the oxide of silicon. In the oxide of silicon having astructure with dispersed silicon crystallites, a diffraction peakassignable to Si (111) is observed near 2θ=28.4°, when a powder X-raydiffraction (XRD) analysis is performed. In a case in which siliconcrystallites are present in an oxide of silicon, it becomes easier toachieve higher initial discharge capacity and higher initial charge anddischarge efficiency.

The crystallite size of silicon is preferably 8 nm or less, and morepreferably 6 nm or less. When the crystallite size is 8 nm or less, asilicon crystallite is not apt to localize in an oxide of silicon, and alithium ion can diffuse easily in an oxide of silicon so as tofacilitate achievement of excellent discharge capacity.

The crystallite size of silicon is preferably 2 nm or more, and morepreferably 3 nm or more. When the crystallite size is 2 nm or more, areaction between a lithium ion and an oxide of silicon can be controlledso as to facilitate achievement of excellent charge and dischargeefficiency.

The crystallite size of silicon can be determined using the Scherrerequation based on the half width of a diffraction peak near 2θ=28.4°assignable to Si (111) obtained by a powder X-ray diffraction analysisusing a radiation source of the CuKα line having a wavelength of0.154056 nm.

A structure, in which silicon crystallites are dispersed in an oxide ofsilicon, can be formed, for example, by heat-treating an oxide ofsilicon in an inert atmosphere in a temperature range from 700° C. to1300° C. to allow disproportionation. Further, it may be formed byadjusting the heating temperature at a heat treatment for adding carbonto an oxide of silicon as described below. The higher the heatingtemperature at the heat treatment becomes, and the longer the heatingtime becomes, the larger the silicon crystallite size tends to become.

When a lump of an oxide of silicon in a size of several cm square isprepared, it should preferably be milled and classified. More precisely,the oxide should be preferably subjected first to primary crushing to asize allowing supply to a pulverizing mill and classification, and thento secondary milling by a pulverizing mill. The average particle size ofthe product particle of an oxide of silicon of the secondary milling ispreferably from 0.1 μm to 20 μm according to a desired final size of thenegative electrode material, and more preferably from 0.5 μm to 10 μm.The average particle size is a diameter at 50% cumulative volume of aparticle size distribution (D50%). This holds true for an expression ofan average particle size below. For measuring an average particle size,a heretofore known method such as a laser diffraction particle sizedistribution analyzer may be used.

A negative electrode material according to the invention contains carbonover a part or a whole of a surface of an oxide of silicon, wherein thecontent of the carbon with respect to a whole negative electrodematerial is from 0.5 mass-% to less than 5 mass-%. With such aconstitution, the initial discharge capacity and the initial charge anddischarge efficiency are improved. The carbon content with respect to awhole negative electrode material is preferably from 0.5 mass-% to 4.5mass-%, and more preferably from 0.5 mass-% to 4.0 mass-%.

The carbon content (by mass) in a whole negative electrode material canbe determined by a microwave calcination-infrared analysis method. For amicrowave calcination-infrared analysis method, for example, acarbon/sulfur determinator (CSLS600, made by Leco Japan Corporation) maybe used.

A negative electrode material according to the invention contains carbonover a part or a whole of a surface of an oxide of silicon. FIG. 1 toFIG. 4 are schematic cross-sectional views showing examples of theconstitution of a negative electrode material according to theinvention. In FIG. 1 carbon 10 coats the whole surface of an oxide ofsilicon 20. In FIG. 2 carbon 10 coats the whole surface of an oxide ofsilicon 20, but does not cover it uniformly. In FIG. 3, carbon 10 ispresent partially on the surface of an oxide of silicon 20, and a partof the surface of the oxide of silicon 20 is exposed. In FIG. 4,particles of carbon 10 having a particle size less than an oxide ofsilicon 20 are present on the surface of an oxide of silicon 20. FIG. 5is a variation of FIG. 4, in which the particle shape of carbon 10 issquamous. Although the shape of an oxide of silicon 20 is depictedschematically spherical (the cross-sectional shape is circular) in FIG.1 to FIG. 5, it may have any of spherical, blockish, squamous, orcross-sectionally polygonal (angulated) shapes.

FIG. 6A and FIG. 6B are enlarged cross-sectional views of a part of thenegative electrode material according to FIG. 1 to FIG. 3, and FIG. 6Aillustrates an embodiment of the shape of carbon 10 in a negativeelectrode material and FIG. 6B illustrates another embodiment of theshape of carbon 10 in a negative electrode material. In the cases inFIG. 1 to FIG. 3, the carbon 10 may be entirely constituted with carbonas shown in FIG. 6A, or the carbon 10 may be constituted with fineparticles 12 as shown in FIG. 6B. Although FIG. 6B depicts a state wherethe contour of a fine particle 12 remains intact, the fine particles 12may be bonded each other. When the fine particles 12 are bonded eachother, the carbon 10 may be entirely constituted with carbon, or voidsmay be included in a part of the carbon 10. Namely, the carbon 10 maypartly include voids.

When the carbon 10 is particles, the particles of the carbon 10 may bepresent only on a part of the surface of an oxide of silicon 20 andanother part of the surface of an oxide of silicon 20 may be exposed asshown in FIG. 4, or the particles of the carbon 10 may be present overthe entire surface of an oxide of silicon 20 as shown in FIG. 6B.

The carbon is preferably low crystalline. Low crystallinity means thatthe following R value is 0.5 or more.

Defining the intensity of a peak appearing near 1360 cm⁻¹ as Id, theintensity of a peak appearing near 1580 cm⁻¹ as Ig in a profile obtainedby laser Raman spectrometry with an excitation wavelength of 532 nm, andthe intensity ratio of both the peaks Id/Ig (expressed also as D/G) as Rvalue, the carbon should preferably has an R value from 0.5 to 1.5, morepreferably from 0.7 to 1.3, and further preferably from 0.8 to 1.2.

When R value is 0.5 or more, high discharge capacity tends to beobtainable, and when it is 1.5 or less, increase in irreversiblecapacity tends to be suppressible.

In this regard, a peak appearing near 1360 cm⁻¹ is ordinarily identifiedas a peak assignable to an amorphous carbon structure, and means, forexample, a peak observed between 1300 cm⁻¹ and 1400 cm⁻¹; while a peakappearing near 1580 cm⁻¹ is ordinarily identified as a peak assignableto a crystal graphite structure, and means, for example, a peak observedbetween 1530 cm⁻¹ and 1630 cm⁻¹.

R value can be determined by a Raman spectrum analyzer (e.g. NSR-1000Model with excitation wavelength 532 nm, made by Jasco Corporation) withrespect to a measurement range from 830 cm⁻¹ to 1940 cm⁻¹ based on abaseline between 1050 cm⁻¹ and 1750 cm⁻¹.

Although there is no particular restriction on a method for addingcarbon onto the surface of an oxide of silicon, and examples thereofinclude a wet mixing method, a dry mixing method, and a chemical vapordeposition method. From viewpoints of homogeneity, easier regulation ofa reaction system, and preservation of a negative electrode materialshape, a wet mixing method or a dry mixing method is preferable.

In the case of a wet mixing method, carbon can be added onto the surfaceof an oxide of silicon, for example, by mixing an oxide of silicon and asolution dissolving a carbon source in a solvent so as to stick thecarbon source solution to the surface of an oxide of silicon, ifnecessary removing the solvent, and then preforming a heat treatment inan inert atmosphere to carbonize the carbon source. In a case in which acarbon source cannot be dissolved in a solvent, the carbon source may bealso dispersed in a dispersing medium to form a dispersion liquid.

In the case of a dry mixing method, carbon can be added onto the surfaceof an oxide of silicon, for example, by mixing two solids of an oxide ofsilicon and a carbon source to form a mixture, and then heat-treatingthe mixture in an inert atmosphere to carbonize the carbon source. Whenan oxide of silicon and a carbon source are mixed, a treatment foradding mechanical energy (for example, a mechanochemical treatment) maybe performed.

In the case of a chemical vapor deposition method, a publicly knownmethod may be applied for adding carbon onto the surface of an oxide ofsilicon, for example, by heat-treating an oxide of silicon in anatmosphere containing a gas prepared by vaporizing a carbon source.

There is no particular restriction on the carbon source, insofar as itis a compound from which carbon can be remained after a heat treatment,when carbon is added onto the surface of an oxide of silicon by themethods, and specific examples thereof include polymers, such as aphenolic resin, a styrenic resin, poly(vinyl alcohol), poly(vinylchloride), poly(vinyl acetate), and poly(butyral); pitches, such asethylene heavy end pitch, coal pitch, petroleum pitch, coal tar pitch,asphalt cracking pitch, PVC pitch formed by thermal decomposition ofpoly(vinyl chloride), etc., and naphthalene pitch formed bypolymerization of naphthalene, etc. in the presence of a super strongacid; and polysaccharides, such as starch, and cellulose. The carbonsources may be used singly, or in a combination of 2 or more kinds.

In a case in which carbon is added by a chemical vapor depositionmethod, a gaseous or an easily vaporizable compound among an aliphatichydrocarbon, an aromatic hydrocarbon, an alicyclic hydrocarbon, and thelike is preferably used as a carbon source. Specific examples thereofinclude methane, ethane, propane, toluene, benzene, xylene, styrene,naphthalene, cresol, anthracene, and derivatives thereof. The carbonsources may be used singly, or in a combination of 2 or more kinds.

There is no particular restriction on a heat treatment temperature forcarbonizing a carbon source, insofar as a carbon source can becarbonized at the temperature, and it is preferably 700° C. or higher,more preferably 800° C. or higher, and further preferably 900° C. orhigher. From viewpoints of making carbon low crystalline and forming thesilicon crystallite having a desired size, the temperature is preferably1300° C. or lower, more preferably 1200° C. or lower, and furtherpreferably 1100° C. or lower.

A heat treatment time may be selected appropriately according to thetype of a carbon source or the addition amount thereof, and it ispreferably, for example, from 1 hour to 10 hours, and more preferablyfrom 2 hours to 7 hours.

A heat treatment is preferably carried out in an inert atmosphere, suchas nitrogen, and argon. There is no particular restriction on a heattreatment apparatus, insofar as it is a reaction apparatus equipped witha heating mechanism. Examples thereof include a heating apparatus, whichcan be operated by a continuous process, a batch-wise process, etc.Specifically, it can be selected appropriately according to an aim froma fluidized bed reaction oven, a rotary oven, a vertical moving bedreaction oven, a tunnel oven, a batch-wise oven, etc.

Since in a heat-treated product obtained from the heat treatmentindividual particles may coagulate together, it is preferable to conducta disintegration treatment. In a case in which adjustment to a desiredaverage particle size is necessary, a grinding treatment may be furtherperformed.

As an example of another method for adding carbon onto the surface of anoxide of silicon, there is a method using a carbonaceous material, suchas amorphous carbon including soft carbon and hard carbon; and graphite,as carbon to be added onto the surface of an oxide of silicon. By thismethod a negative electrode material configured such that carbon 10 ispresent as particles over the surface of an oxide of silicon 20 as shownin FIG. 4 and FIG. 5 can be also prepared. As a method for using thecarbonaceous material, the wet mixing method or the dry mixing method asabove can be applied.

In a case in which a wet mixing method is applied, a fine particle of acarbonaceous material and an organic compound (a compound leaving carbonafter a heat treatment) to function as a binder are mixed to form amixture, the mixture and an oxide of silicon are mixed further so thatthe mixture sticks to the surface of the oxide of silicon, which is thenheat-treated to complete production. There is no particular restrictionon the organic compound, insofar as it is a compound which can leavecarbon after a heat treatment. As a heat treatment condition in a casein which a wet mixing method is applied, a heat treatment condition forcarbonizing the carbon source can be applied.

In a case in which a dry mixing method is applied, two solids of a fineparticle of a carbonaceous material and an oxide of silicon are mixedtogether to form a mixture, which is then subjected to a treatment foradding mechanical energy (for example, a mechanochemical treatment) tocomplete production. Also in a case in which a dry mixing method isapplied, it is preferable that a heat treatment is carried out so as toform silicon crystallites in an oxide of silicon. As a heat treatmentcondition in a case in which a dry mixing method is applied, a heattreatment condition for carbonizing the carbon source can be applied.

The volume-based average particle size of a negative electrode materialaccording to the invention is preferably from 0.1 μm to 20 μm, and morepreferably from 0.5 μm to 10 μm. When the average particle size is 20 μmor less, the distribution of a negative electrode material in a negativeelectrode can become homogeneous, and moreover expansion and contractionduring charging and discharging can become uniform, and thereforedecrease in cycle performance tends to be suppressed. Meanwhile, whenthe average particle size is 0.1 μm or more, the negative electrodedensity tends to increase, and higher capacity tends to be available.

The specific surface area of a negative electrode material according tothe invention is preferably from 0.1 m²/g to 15 m²/g, more preferablyfrom 0.5 m²/g to 10 m²/g, and further preferably from 1.0 m²/g to 7m²/g. When the specific surface area is 15 m²/g or less, increase in thefirst irreversible capacity of a product lithium ion secondary batterytends to be suppressed. Further, increase in the consumption of a binderduring producing a negative electrode tends to be suppressed. When thespecific surface area is 0.1 m²/g or more, the contact area with anelectrolyte solution increases and the charge and discharge efficiencytends to increase. For measuring a specific surface area, a heretoforeknown method such as a BET method (a nitrogen gas adsorption method) canbe utilized.

With respect to a negative electrode material according to theinvention, preferably the carbon content is from 0.5 mass-% to less than5 mass-%, and the silicon crystallite size is from 2 nm to 8 nm, andmore preferably the carbon content is from 0.5 mass-% to 4.5 mass-%, andthe silicon crystallite size is from 3 nm to 6 nm.

The negative electrode material may be used, if necessary, together witha heretofore known carbonaceous negative electrode material as an activematerial for a negative electrode of a lithium ion secondary battery.According to the type of a carbonaceous negative electrode material tobe used together, improvement in the charge and discharge efficiency,improvement in the cycle performance, an inhibitory effect on electrodeexpansion, or the like can be obtained.

Examples of a heretofore known carbonaceous negative electrode materialinclude natural graphite, such as squamous natural graphite, sphericalnatural graphite prepared by spherizing squamous natural graphite,artificial graphite, and amorphous carbon. The carbonaceous negativeelectrode material may further contain carbon on a part or a whole ofthe surface. The carbonaceous negative electrode materials may be usedsingly, or in a combination of plural kinds, as mixed with the abovenegative electrode material according to the invention.

When a negative electrode material according to the invention is used ina combination with a carbonaceous negative electrode material, the ratioof a negative electrode material according to the invention (denoted as“SiO-C”) to a carbonaceous negative electrode material (denoted as “C”),namely SiO—C:C, may be adjusted appropriately according to an aim, andit is from a viewpoint of an inhibitory effect on expansion of anelectrode preferably, for example, from 0.1:99.9 to 20:80 by mass, morepreferably from 0.5:99.5 to 15:85, and further preferably from 1:99 to10:90.

<Negative Electrode for Lithium Ion Secondary Battery>

A negative electrode for a lithium ion secondary battery according tothe invention (hereinafter occasionally abbreviated as “negativeelectrode”) includes a current collector, and a negative electrodematerial layer provided on the current collector and containing thenegative electrode material for a lithium ion secondary battery. Anegative electrode for a lithium ion secondary battery according to theinvention is prepared, for example, by mixing the negative electrodematerial for a lithium ion secondary battery, an organic binder, adissolving medium, such as a solvent and water, as well as, ifnecessary, a thickener, an electric conduction aid, a heretofore knowncarbonaceous negative electrode material, etc. to prepare a coatingliquid, applying (coating) the coating liquid onto a current collector,then removing the solvent or water, and pressing to form a negativeelectrode material layer. The material is generally kneaded with anorganic binder, a solvent, etc. and formed to a sheet or pellets.

Although there is no particular restriction on the organic binder,examples thereof include a styrene-butadiene copolymer; a (meth)acryliccopolymer obtained by copolymerizing an ethylenic unsaturated carboxylicacid ester, such as methyl(meth)acrylate, ethyl(meth)acrylate,butyl(meth)acrylate, (meth)acrylonitrile, andhydroxyethyl(meth)acrylate, and an ethylenic unsaturated carboxylicacid, such as acrylic acid, methacrylic acid, itaconic acid, fumaricacid, and maleic acid; and a polymer, such as poly(vinylidene fluoride),poly(ethylene oxide), polyepichlorohydrin, polyphosphazene,polyacrylonitrile, polyimide, and polyamide-imide. Meanwhile, a“(meth)acrylate” means an “acrylate” and an “methacrylate” correspondingthereto. This holds true for a similar expression such as “(meth)acryliccopolymer”.

Some of the organic binders are dispersed or dissolved in water and someothers are dissolved in an organic solvent such asN-methyl-2-pyrrolidone (NMP) depending on the respective physicalproperties. Among others, an organic binder, whose main skeleton ispolyacrylonitrile, polyimide or polyamide-imide is preferable from aviewpoint of superior adherence, and an organic binder whose mainskeleton is polyacrylonitrile is more preferable from viewpoints of alow heat treatment temperature during production of a negative electrodeand superior electrode flexibility as described below. As an organicbinder whose main skeleton is polyacrylonitrile, for example, a product(LSR7 (trade name), made by Hitachi Chemical Co., Ltd., etc.), in whichacrylic acid imparting adherence and a straight chain ether groupimparting flexibility are added to a polyacrylonitrile skeleton, can beused.

The content of an organic binder in a negative electrode material layerof a negative electrode material for a lithium ion secondary battery ispreferably from 0.1 mass-% to 20 mass-%, more preferably from 0.2 mass-%to 20 mass-%, and further preferably from 0.3 mass-% to 15 mass-%.

When the content of an organic binder is 0.1 mass-% or more, theadherence is superior, and breakage of a negative electrode due toexpansion and contraction during charging and discharging tends to besuppressed. Meanwhile, when the content is 20 mass-% or less, increasein electrode resistance tends to be suppressed.

Further, as a thickener for adjusting the viscosity, carboxymethylcellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose,poly(vinyl alcohol), poly(acrylic acid) (acrylate), oxidized starch,phosphorylated starch, casein, or the like may be used together with theorganic binder.

There is no particular restriction on a solvent to be used for mixing anorganic binder, and examples thereof include N-methylpyrrolidone,dimethylacetamide, dimethylformamide, and γ-butyrolactone.

To the coating liquid an electric conduction aid may be added. Examplesof the electric conduction aid include carbon black, acetylene black, anoxide having electroconductivity, and a nitride havingelectroconductivity. The electric conduction aids may be used singly, orin a combination of 2 or more kinds. The content of an electricconduction aid is preferably from 0.1 mass-% to 20 mass-% with respectto a negative electrode material layer (100 mass-%).

There is no particular restriction on the material for a currentcollector, and examples thereof include aluminum, copper, nickel,titanium, stainless steel, a porous metal (metal foam), and carbonpaper. There is no particular restriction on the form of a currentcollector, and examples thereof include a foil form, a perforated foilform, and a mesh form.

There is no particular restriction on a method for applying (coating)the coating liquid onto a current collector, and examples thereofinclude a metal mask printing method, an electrostatic coating method, adip coating method, a spray coating method, a roll coating method, adoctor blade method, a gravure coating method, and a screen printingmethod. After coating, a pressure treatment is preferably performed by aflat plate press, a calender roll, or the like according to need.

Integration of a coating liquid formed into a sheet form, a pellet form,or the like and a current collector may be conducted by integration byrolling, or integration by pressing, or integration by a combination ofthe two.

A negative electrode material layer formed on a current collector, or anegative electrode material layer integrated with a current collector ispreferably heat-treated depending on the organic binder used. Forexample, in a case in which an organic binder with a main skeleton ofpolyacrylonitrile is used, a heat treatment is conducted preferably atfrom 100° C. to 180° C., and in a case in which an organic binder with amain skeleton of polyimide or polyamide-imide is used, a heat treatmentis conducted preferably at from 150° C. to 450° C.

By the heat treatment, the strength is highly intensified throughremoval of a solvent and curing of an organic binder, and internaladherence in a negative electrode material and adherence between anegative electrode material and a current collector can be improved. Theheat treatment is preferably carried out in an inert atmosphere, such ashelium, argon, and nitrogen, or in a vacuum atmosphere, in order toprotect a current collector from oxidation during the treatment.

A negative electrode is preferably pressed (pressure-treated) prior to aheat treatment. By a pressure treatment the electrode density can beadjusted. The electrode density of a negative electrode for a lithiumion secondary battery according to the invention is preferably from 1.4g/cm³ to 1.9 g/cm³, more preferably from 1.5 g/cm³ to 1.85 g/cm³, andfurther preferably from 1.6 g/cm³ to 1.8 g/cm³. The higher the electrodedensity is, the more the volumetric capacity of a negative electrodetends to be improved, and the more the internal adherence in a negativeelectrode material and the adherence between a negative electrodematerial and a current collector tend to be improved.

<Lithium Ion Secondary Battery>

A lithium ion secondary battery according to the invention is providedwith a positive electrode, the negative electrode, and an electrolyte.

By placing the negative electrode, for example, facing to a positiveelectrode intercalating a separator, and by injecting therein anelectrolytic solution containing an electrolyte, a lithium ion secondarybattery can be constituted.

The positive electrode can be obtained similarly as the negativeelectrode by forming a positive electrode layer on the surface of acurrent collector. As a current collector for the positive electrode, acurrent collector similar to those described for the negative electrodecan be used.

There is no particular restriction on a material to be used for apositive electrode of a lithium ion secondary battery according to theinvention (also referred to as “positive electrode material”), insofaras it is a compound, which can be doped or intercalated with a lithiumion, and examples thereof include lithium cobaltate (LiCoO₂), lithiumnickelate (LiNiO₂), and lithium manganate (LiMnO₂).

A positive electrode can be prepared, for example, by mixing thepositive electrode material, an organic binder such as poly(vinylidenefluoride), and a solvent, such as N-methyl-2-pyrrolidone, andγ-butyrolactone to prepare a positive electrode coating liquid, applying(coating) the positive electrode coating liquid onto at least onesurface of a current collector such as an aluminum foil, then removingthe solvent by drying, and, if necessary, performing a pressuretreatment.

To a positive electrode coating liquid an electric conduction aid may beadded. Examples of the electric conduction aid include carbon black,acetylene black, an oxide having electroconductivity, and a nitridehaving electroconductivity. The electric conduction aids may be usedsingly, or in a combination of 2 or more kinds.

There is no particular restriction on an electrolyte solution to be usedfor a lithium ion secondary battery according to the invention, and apublicly known solution may be used. A nonaqueous lithium ion secondarybattery can be produced, for example, using a solution in which anelectrolyte is dissolved in an organic solvent as an electrolytesolution.

Examples of an electrolyte include LiPF₆, LiClO₄, LiBF₄, LiClF₄, LiAsF₆,LiSbF₆, LiAlO₄, LiAlCl₄, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, LiC(CF₃SO₂)₃,LiCl, and LiI.

There is no particular restriction on the organic solvent, insofar as itcan dissolve the electrolyte, and examples thereof include propylenecarbonate, ethylene carbonate, diethyl carbonate, ethyl methylcarbonate, vinyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, and2-methyltetrahydrofuran.

As a separator, various publicly known separators can be also used.Specific examples include a paper separator, a polypropylene separator,a polyethylene separator, and a glass fiber separator.

According to an exemplary production process for a lithium ion secondarybattery: two electrodes of a positive electrode and a negative electrodeare wound up intercalating a separator; the obtained spirally wound-upbody is inserted into a battery can; a tab terminal welded in advance toa current collector of the negative electrode is welded to the batterycan bottom; an electrolyte solution is injected into the thus preparedbattery can; and a tab terminal welded in advance to a current collectorof the positive electrode is welded to a battery cover, which is thenplaced on top of the battery can intercalating an insulating gasket,followed by caulking of contact parts between the cover and the batterycan for hermetical closure, thereby completing a battery.

There is no particular restriction on the shape of a lithium ionsecondary battery according to the invention, and examples of a lithiumion secondary battery include a paper battery, a button battery, a coinbattery, a layered battery, a cylindrical battery, and a rectangularbattery.

Although a negative electrode material for a lithium ion secondarybattery according to the invention is described as “for a lithium ionsecondary battery”, it can be applied to any and all electrochemicaldevices with a charge and discharge mechanism, in which a lithium ion isincluded and eliminated.

EXAMPLES

The invention will be described more specifically below by way of asynthesis example, Examples, and Comparative Examples, provided that theinvention be not limited to the following Examples. Meanwhile, “part(s)”and “%” are by mass, unless otherwise specified.

Example 1 Production of Negative Electrode Material

Massive silicon oxide (10 mm to 30 mm-square, made by Kojundo ChemicalLab. Co., Ltd.) as an oxide of silicon was coarsely ground in a mortarto obtain a particle of an oxide of silicon. The particle of an oxide ofsilicon was ground further by a vibration mill (small size vibrationmill NB-0, made by Nitto Kagaku Co., Ltd.), and the particle size isadjusted by a test sieve 300M (300 mesh) to obtain a fine particle withan average particle size of 5 μm.

<Measurement of Average Particle Size>

A measurement sample (5 mg) was placed in a 0.01 mass-% aqueous solutionof a surfactant (ETHOMEEN T/15, made by Lion Corporation) and dispersedwith a vibration stirrer. The obtained dispersion liquid was placed in asample water tank of a laser diffraction particle size distributionanalyzer (SALD3000J, made by Shimadzu Corporation) and a measurement wascarried out by a laser diffraction method with circulation by a pumpwhile applying ultrasonic waves. The measurement conditions were asfollows. A diameter at 50% cumulative volume of the obtained particlesize distribution (D50%) was defined as an average particle size.Measurements of average particle sizes in the following Examples wereconducted identically.

Light source: red semiconductor laser (690 nm)

Absorbance: 0.10 to 0.15

Refractive index: 2.00-0.20i

Into a mixing apparatus (ROCKING MIXER RM-10G, made by Aichi ElectricCo., Ltd.), 995 g of the obtained fine particles of an oxide of siliconand 10 g of coal pitch (fixed carbon 50 mass-%) were charged, mixed for5 min, and filled in an alumina-made heat treatment container. Thefilled heat treatment container was heat-treated in a controlledatmosphere baking furnace in a nitrogen atmosphere at 1000° C. for 5hours to obtain a heat-treated product.

The obtained heat-treated product was disintegrated in a mortar andsieved out by a test sieve of 300M (300 mesh) to obtain a negativeelectrode material.

<Measuring Method of Carbon Content>

The carbon content of the negative electrode material was measured by amicrowave calcination-infrared analysis method. A microwavecalcination-infrared analysis method is an analysis method by which asample is heated to be calcined in an oxygen flow in a microwavefurnace, so that carbon and sulfur in the sample are converted to CO₂and SO₂ respectively and then analyzed quantitatively by an infraredabsorption method. A measuring apparatus and measurement conditions,etc. are as follows.

Apparatus: Carbon/sulfur determinator (CSLS600, made by Leco JapanCorporation)

Frequency: 18 MHz

Microwave output: 1600 WSample mass: approx. 0.05 gAnalysis time: Automated mode was selected in Setting mode of theapparatus.Burning improver: Fe+W/Sn Standard sample: LECO 501-024 (C: 3.03%±0.04,S: 0.055%±0.002)Number of measurements: 2 times (A value of carbon content in Table 2 isan average value of 2 measured values.)

<Measurement of R Value>

From a spectrum obtained using a Raman spectrometer (NSR-1000 Model,made by Jasco Corporation), the negative electrode material was analyzedbased on a baseline within the following range. Measurement conditionswere as follows.

Laser wavelength: 532 nmIrradiation intensity: 1.5 mW (value measured by a laser power monitor)Irradiation time: 60 secIrradiation area: 4 μm²Measurement range: 830 cm⁻¹ to 1940 cm⁻¹Baseline: 1050 cm⁻¹ to 1750 cm⁻¹

The wave number of an obtained spectrum was corrected using acalibration curve obtained from differences between wave numbers ofrespective peaks found by a measurement under the same conditions with areference material of indene (E. P. grade: Wako Pure ChemicalIndustries, Ltd.) and theoretical wave numbers of the respective peaksof indene.

Defining the intensity of a peak appearing near 1360 cm⁻¹ as Id, theintensity of a peak appearing near 1580 cm⁻¹ as Ig in a profile obtainedafter the correction, and the intensity ratio of both the peaks Id/Ig(D/G) was determined as R value.

<Measurement of BET Specific Surface Area>

Nitrogen adsorption was measured by a 5-point method at a liquidnitrogen temperature (77K) using an accelerated surface area andporosimeter (ASAP2020, made by Micromeritics Instrument Corporation),and a specific surface area was calculated by a BET method (relativepressure range: from 0.05 to 0.2).

<Measurement of Silicon Crystallite Size>

The negative electrode material was analyzed using a powder X-raydiffractometer (MULTIPLEX (2 kW), made by Rigaku Corporation). Thesilicon crystallite size was calculated from the half width of a peakassignable to the crystal face of Si (111) present near 2θ=28.4° usingthe Scherrer equation. Measurement conditions were as follows.

Radiation source: CuKα line (wavelength: 0.154056 nm)Measurement range: 2θ=10° to 40°Sampling step width: 0.02°Scanning speed: 1°/minTube current: 40 mATube voltage: 40 kVDivergence slit: 1°Scattering slit: 1°Receiving slit: 0.3 mm

An obtained profile was subjected to removal of background (BG) andseparation of a peak using a structural analysis software attached tothe apparatus (JADE6, made by Rigaku Corporation) with the followingsetting.

[Removal of Kα2 Peak and Removal of Background]

Intensity ratio Kα1/Kα2: 2.0Offset (σ) of BG curve from BG dot: 0.0

[Designation of Peak]

Peak assignable to Si (111): 28.4°±0.3°Peak assignable to SiO₂: 21°±0.3°

[Peak Separation]

Profile form function: Pseudo-Voigt

Background Fixed

By reading the half width of a peak assignable to Si (111) derived bythe structural analysis software with the above setting, a siliconcrystallite size was calculated by the following Scherrer equation.

D=Kλ/B cos θ

B=(B_(obs) ²−b²)^(1/2)D: Crystallite size (nm)K: Scherrer constant (0.94)λ: Radiation source wavelength (0.154056 nm)θ: Found half width of peak angleB_(obs): Half width (Found value obtained from structural analysissoftware)b: Found half width of standard silicon (Si)

(Production Method of Negative Electrode)

To 3.75 mass-% of a powder of the negative electrode material producedby the above technique and 71.25 mass-% of artificial graphite (made byHitachi Chemical Co., Ltd.) as a carbonaceous negative electrodematerial (produced negative electrode material:artificial graphite=5:95(mass ratio)), 15 mass-% of a powder of acetylene black (made by DenkiKagaku Kogyo K.K.) as an electric conduction aid, and LSR-7 (made byHitachi Chemical Co., Ltd.) as a binder were added, and then the mixturewas kneaded to prepare a homogeneous slurry. In this case, the additionamount of the binder was adjusted to 10 mass-% with respect to the totalmass of the slurry. The slurry was coated on a glossy surface of anelectrolytic copper foil to a coating amount of 10 mg/cm², which wasthen pre-dried at 90° C. for 2 hours and adjusted to a density of 1.65g/cm³ by a roll press. The above was then dried in a vacuum atmosphereat 120° C. for 4 hours for performing a curing treatment to complete anegative electrode.

(Production of Lithium Ion Secondary Battery)

A 2016 type coin cell was produced using the electrode produced above asa negative electrode, a metallic lithium as a counter electrode, amixture liquid of ethylene carbonate/ethyl methyl carbonate (volumeratio=3:7) and vinyl carbonate (VC) (1.0 mass-%) containing 1 M of LiPF₆as an electrolyte solution, a 25 μm-thick polyethylene microporousmembrane as a separator, and a 250 μm-thick copper plate as a spacer.

(Battery Evaluation) <First Discharge Capacity, Charge and DischargeEfficiency>

A battery produced as above was placed in a thermostatic chamber kept at25° C., a constant current charging was carried out at 0.43 mA (0.32mA/cm²) to reach 0 V, then a constant voltage charging was furthercarried out at 0 V until the current attenuated to a value correspondingto 0.043 mA, and the first battery charge capacity was measured. After arest for 30 min from the completion of charging, the battery wasdischarged, such that a discharge at 0.43 mA (0.32 mA/cm²) was carriedout down to 1.5 V and the first discharge capacity was measured. In thiscase, the capacity was reduced to a value per mass of a negativeelectrode material (total mass of a mixture of a produced negativeelectrode material and artificial graphite). The initial charge anddischarge efficiency (%) was calculated as a value obtained by dividingthe first discharge capacity by the first battery charge capacity.

Examples 2 to 6, Comparative Examples 2 and 3

A negative electrode material was produced identically with productionof the negative electrode material in Example 1, except that thecontents of an oxide of silicon and coal pitch were changed as in thefollowing Table, and a similar evaluation was carried out.

TABLE 1 Oxide of silicon [g] Coal pitch [g] Example 2 990 20 Example 3980 40 Example 4 970 60 Example 5 960 80 Example 6 955 90 ComparativeExample 2 950 100 Comparative Example 3 920 160

Comparative Example 1

A negative electrode material was produced identically with productionof the negative electrode material in Example 1, except that pitch wasnot mixed and only an oxide of silicon was heat-treated, and a similarevaluation was carried out. The evaluation results with respect toExamples and Comparative Examples are shown in the following Table 2.

TABLE 2 Comparative Example Example Example Example Example ExampleComparative Comparative Example 1 1 2 3 4 5 6 Example 2 Example 3 Carbon0.0 0.5 1.0 2.0 3.0 4.0 4.5 5.0 8.0 content [mass-%] R value — 1.1 1.11.0 0.9 0.9 1.0 0.9 1.0 (D/G) BET specific 1.8 2.0 2.1 2.0 2.5 2.8 3.03.4 5.2 surface area [m²/g] Average 5.0 5.0 5.0 5.5 5.5 6.0 6.0 6.0 6.5particle size [μm] Silicon 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0crystallite size [nm] First 419 450 450 450 447 446 446 444 444 batterycharge capacity [mAh/g] First 378 405 406 406 403 402 402 398 397discharge capacity [mAh/g] Charge and 90.2 90 90.2 90.2 90.2 90.1 90.189.6 89.4 discharge efficiency [%]

As obvious from the results in Table 2, negative electrode materials fora lithium ion secondary battery shown in Examples 1 to 6 are materialshaving a higher first discharge capacity and superior in initial chargeand discharge efficiency compared to Comparative Example 1 withoutcarbon coating and Comparative Examples 2 and 3 having a carbon coatamount of 5 mass-% or more.

In a case in which only artificial graphite was used as a negativeelectrode material, the first battery charge capacity was 378 mAh/g, andthe first discharge capacity was 355 Ah/g. Compared to the results ofthis case in which only artificial graphite was used, in Examples, inwhich a negative electrode material contains 5 mass-% of a negativeelectrode material according to the invention and 95 mass-% ofartificial graphite, despite such a low content of a negative electrodematerial according to the invention, it is obvious that the firstbattery charge capacity as well as discharge capacity are improvedremarkably.

The entire contents of the disclosures by Japanese Patent ApplicationNo. 2012-237256 are incorporated herein by reference.

All the literature, patent literature, and technical standards citedherein are also herein incorporated to the same extent as provided forspecifically and severally with respect to an individual literature,patent literature, and technical standard to the effect that the sameshould be so incorporated by reference.

1. A negative electrode material for a lithium ion secondary battery,comprising carbon over a part or a whole of a surface of an oxide ofsilicon, wherein the content of the carbon is from 0.5 mass-% to lessthan 5 mass-%.
 2. The negative electrode material for a lithium ionsecondary battery according to claim 1, wherein the carbon comprises lowcrystallinity carbon.
 3. The negative electrode material for a lithiumion secondary battery according to claim 1, wherein a diffraction peakassignable to Si (111) is observed when the negative electrode materialis subjected to a powder X-ray diffraction (XRD) analysis.
 4. A negativeelectrode for a lithium ion secondary battery, comprising: a currentcollector; and a negative electrode material layer provided on thecurrent collector and comprising the negative electrode materialaccording to claim
 1. 5. A lithium ion secondary battery, comprising: apositive electrode; the negative electrode for a lithium ion secondarybattery according to claim 4; and an electrolyte.